1. Field of the invention
The invention relates to a forming device for an extrusion plant having at least one calibrating device comprising at least one calibration tool having a plurality of calibration dies arranged one after the other in the extrusion direction, wherein the calibration dies have at least one calibration orifice with several forming surfaces which are applied against at least one object fed through, in particular a section with several cavities, as well as end faces perpendicular thereto and spaced at a distance apart from one another in the extrusion direction extending parallel with one another, with side faces extending between them, wherein the first end face is disposed facing an inlet region and the second end face faces an outlet region for the object fed through and having a cavity between at least two immediately adjacent calibration dies extending from the forming surface through to a passage and opening therein and means co-operating with the calibration device to carry away heat.
2. The Prior Art
A forming device for an extrusion plant is known from U.S. Pat. No. 4,181,487 A1, which has at least one sizing tool with a plurality of side-by-side sizing dies of a plate-type design. The individual side-by-side sizing dies have front faces extending more or less perpendicular or normal to the forming surfaces, channels being formed in at least one of the front faces, extending from the forming surfaces across a distribution passage through to a collection passage. When the sizing dies are placed one against the other, these passages form a plurality of cavities, by means of which a pressure differential can be generated in the object to be fed through. These individual sizing dies co-operate with a plurality of hollow components through which a coolant medium is circulated to carry the heat picked up from the sizing dies co-operating therewith, in the region of their external front faces, away from the object. The disadvantage of this design of sizing dies is that it is not always possible to satisfactorily size the object to be cooled in all applications.
Other forming devices for extrusion plants are known from DE 199 17 837 A1, and from DE 297 16 343 U1 filed by the same applicant, which have at least one calibration device having in particular a plurality of calibration tools disposed one after the other in the extrusion direction, this calibration device and/or the calibration tools being provided with cooling passages through which a coolant medium is circulated. The calibration tool also has forming surfaces which are applied against an object fed through, a seal being disposed at least between the extrusion tool, e.g. a nozzle, of the forming device and the first immediately adjacent calibration tool in the extrusion device so as to form a cavity, sealing the external surface of the object to be fed through from the ambient pressure. However, such a seal may also be disposed between the first calibration tool and at least another calibration tool to form another cavity. These calibration tools are of a building block design and it has not always proved possible to obtain satisfactory calibration results with this embodiment.
Forming devices are also known in which the plastic strands of sections leaving an extrusion nozzle, in particular hollow sections or tubes, are sized to desired external and internal dimensions and frozen in the desired external dimensions or cooled to a pre-set temperature to produce the desired inherent rigidity. Forming devices of this type are described in the book xe2x80x9cExtrusionswerkzeuge fxc3xcr Kunststoffe und Kautschukxe2x80x9d by Walter Michaeli published in 1991 by Carl Hanser Verlag, Munich/Vienna, 2nd fully revised and extended edition, in particular pages 321 to 329. Page 323 illustrates an example of a vacuum-sizing process used to produce the external dimensions in which the extruded material leaving the extrusion tool in the form of a hollow section is fed into a calibration tool at a distance downstream of the outlet of the nozzle lip of the extrusion tool, by means of which the external face of the extruded material is applied against the forming surfaces of the calibration tool. The surface of the extruded material is applied against the individual forming surfaces of the calibration tool without any clearance by means of air vents, in particular slits, disposed in the forming surfaces, connected to a vacuum source by means of supply lines. In order to cool the extruded material, the calibration tools are provided with one or more circumferential passages for a coolant medium and the vacuum may be increased in the individual air vents or slits at an increasing distance from the nozzle lip of the extrusion tool. The temperature of the coolant medium is very low relative to the mass temperature of the extruded section and is approximately 20xc2x0 C. Adjoining this dry calibrator, one or more cooling baths are generally provided, in which the sections can be brought to a sufficient rigidity, partially by applying a vacuum and wetting by means of spray nozzles or by feeding them through water baths at room temperature. In many cases, the surface quality of the extruded materials produced with calibration tools of this type is not satisfactory during the later service life of the forming device.
The underlying objective of this invention is to propose a forming device, in particular a calibration tool, in which an almost uniform vacuum pressure can be applied to the object whose external surface is to be sized and cooled, thereby simultaneously reducing the complexity of the apparatus used for production purposes.
This objective is achieved by the invention, in particular, in that the cavity or the cavities is or are open across a predominant part of the periphery of the calibration orifice towards the calibration orifice bounded by the forming surfaces. The surprising advantage of this solution is that a cavity or a plurality of cavities is arranged across the greater part of the circumference of the calibration orifice as far as the forming surfaces bounding the calibration orifice, and because of the surrounding cavity, a uniformly homogeneous vacuum can be produced across almost the entire circumference of the section. Another advantage is that a pressure below ambient pressure can be applied, even in contoured sections which are difficult to calibrate, thereby producing a perfect calibration result in these regions. At the same time, because the pressure differential between the cavities of the hollow section and the external surfaces of the object is uniform, the external surfaces of the object can be applied almost fully against the forming surfaces, thereby enhancing the dimensional accuracy of the object to be produced. Because the cavity is designed to run around the circumference of the calibration orifice, the complexity of the production apparatus can be reduced since, if it were of the building bock design, vacuum slits would have to be provided in the die blocks as well as bores to enable a vacuum to be applied to the vacuum slits.
Also of advantage is another embodiment, because the volume of the cavity increases the farther it is away from the forming surfaces, leaving a larger cross section from which air can be sucked out to build up the pressure and, with longer suction paths, enabling losses in pressure to be compensated so that almost homogeneous vacuum can be built up in the region of the external surface of the object.
Other advantages are that the width of the cavity can be selected so that a corresponding flow cross section can be set and, in the cross-over region between immediately facing end faces of calibration dies disposed immediately one after the other, the dimensions of the cavity or the gap between the end faces can be duly selected so that, firstly, any losses in flow can be compensated and, secondly, too excessive a suction on the surface parts of the object through the vacuum slit can be prevented.
Another advantage is that the pressure differential between the cavity of the object and the external surface thereof can be more accurately set.
Feed-through of the object into the individual calibration dies can be improved and made smoother in that there is a radius of between 0.1 mm and 1.0 mm in the transition region between the forming surfaces and the immediately adjacent first part end faces of the calibration dies.
Other advantageous embodiments of the forming device, in particular the calibration dies, are also described and the advantages achieved can be found in the detailed description relating to the drawings.
There are also advantages to be had from other embodiments, since these provide an improved and more efficient dispersion of heat from the object to the calibration dies. Because additional bores running through to the individual vacuum slits can be dispensed with, the flow orifices to the forming surfaces of the calibration orifice can be positioned more accurately, these flow orifices extending through the calibration tools in the extrusion device.
As a result of other advantageous embodiments, a predeterminable quantity of heat can be fed away from profiled sections of the object which are difficult to cool and the pressure differential generated in certain regions can be varied in a simple manner.
Finally, other advantageous embodiments of the forming device are also described and the advantages achieved can be found in the relevant sections of the description.